The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for using multiband radio frequency (“RF”) pulses in MRI.
The distribution of transverse relaxation times (“T2”) in tissues covers about six orders of magnitude, from microseconds (e.g., for bound water and macromolecules) up to seconds (e.g., for cerebrospinal fluid). For diagnostic purposes, it is very useful to be able to observe changes of spin density and relaxation parameters due to tissue pathology. For these reasons, there has been much interest in MRI methods that are capable of detecting a wide range of short T2s.
There are at least a four different clinically applicable short T2 sensitive MRI methods, including ultrashort echo time (“UTE”) imaging; sweep imaging with Fourier transformation (“SWIFT”; FID-projection imaging, which may also called BLAST, RUFIS, WASPI, or zero TE (“ZTE”); and ZTE combined with single point imaging (“SPI”), such as the PETRA method. All of these methods utilize a radial readout technique.
To increase the range of detectable short T2s, the excitation and acquisition bandwidths must be increased, and the strength of the frequency-encoding (e.g., readout) gradients must be increased. These requirements are not easily accomplished in practical implementations.
For example, in ZTE and SWIFT sequences, an excitation pulse is applied in presence of readout gradients and, therefore, needs to be broadband enough to cover the entire field-of-view (“FOV”) at the readout bandwidth. An excitation bandwidth of the square pulse used in ZTE is inversely proportional to the length of the pulse; thus, increasing the excitation bandwidth requires increasing the amplitude of the RF pulse, which is limited by RF hardware (e.g., peak amplifier power). SWIFT and UTE are less prone to this restriction. For instance, in SWIFT, longer frequency swept pulses are used. In UTE, the excitation pulse is applied before turning on the gradients and, therefore, the excitation bandwidth needs to cover only the resonance lines of the spins. ZTE has fewer problems during readout relative to UTE or SWIFT, however.
In the UTE sequence the “bottle neck” is the duration of gradient ramping time, which physically cannot be shortened enough due to inductance of gradient coils and eddy currents. At the same time, the ring down time of RF coil restricts using regular SWIFT with a time-shared acquisition at high bandwidths.
Thus for various reasons, the existing pulse sequences for imaging short T2 signals are limited to a maximum achievable bandwidth that may be insufficient to resolve all excited short T2 signals and to avoid off-resonance artifacts (e.g., blurring) in radial imaging.
Thus, there remains a need for providing an imaging sequence capable of imaging a wide range of short T2 signals that also has increased efficiency at high bandwidths relative to other methods.